The present invention relates generally to an uninterruptible power supply (UPS) system for interfacing a critical load with a utility power source and for supplying battery backup power to the critical load during emergency conditions, such as utility power line outages, and more particularly to an improved standby power supply (SPS) system, such as may be used to provide power to computer, communication and medical equipment.
A revolution in the fields of microelectronics technology and the information sciences has led to the widespread use of critical electrical loads, such as personal computers. The communication and medical fields have also advanced with this revolution, with electronic devices now providing crucial services and functions. This trend is expected to continue in the future. Such loads typically operate on single-phase alternating current (AC) power received from an electric utility. During a utility power outage, a battery backup is often required to avoid the loss of critical information, functions or services. A variety of UPS systems have been designed to meet these emergency power needs.
Conventional UPS systems provide an interface between the utility power system input and a critical load (personal computers, communication and medical equipment, and other loads normally receiving AC power and requiring battery backup power will herein be referred to as "critical loads"). Basically, the AC power from the utility is first rectified, and a small portion of the rectified power is used to charge the backup battery. The remaining portion of the rectified power is supplied to the critical load through an inverter, which converts the DC voltage to nearly sinusoidal line-frequency (e.g., 50 Hz in Europe or 60 Hz in the Unites States) voltage, to the critical load. During a utility power outage emergency condition, power is supplied by the battery through the inverter to the load.
Conventional UPS systems typically have a rectifier/charger which receives AC power from an AC power utility through a static interrupter and rectifies it into DC power. A portion of the rectified DC power charges a battery bank during normal operating conditions. A line-frequency isolation transformer is often included between the interrupter and the critical load. During normal operating conditions, the interrupter is closed to allow the AC power to flow therethrough to a critical load.
During a utility power outage, the static interrupter opens to isolate the load from the utility. The battery bank powers the load through a pulse width modulated (PWM) DC-to-AC inverter. Line-frequency isolation transformers may be placed between the inverter and the load. Thus, power is maintained to the load during both normal and emergency operating conditions. Alternatively, during normal operation, power is supplied to the load through the rectifier/charger, the inverter and an isolation transformer. In this alternate embodiment, the AC power supplied through the interrupter and isolation transformer (if used) is merely an additional backup feature to further enhance the reliability of the UPS system.
The known conventional UPS systems suffer several significant drawbacks. For example, the line-frequency isolation transformers are bulky, expensive and introduce substantial power losses into the UPS system.
The conventional UPS systems disadvantageously inject undesirable current harmonics into the utility system. These injected current harmonics produce line voltage distortion which interferes with other loads connected to the same line voltage supply. The rectifier/charger and the inverter are each sources of these undesirable current harmonics. However, the inverter harmonic currents are decoupled from the utility by the rectifier/charger and by using an isolation transformer. An active filtering system to actively neutralize harmonic currents injected by an AC-to-DC converter into an AC power transmission system is disclosed in U.S. Pat. Nos. 4,053,820 and 4,224,660 (the inventor of the present invention being a co-inventor and the sole inventor, respectively, of these patents).
Another significant source of undesirable current harmonics is the critical load itself, which is often some form of a DC power supply. These high technology electrical loads typically have a highly nonlinear input characteristic, that is, the input current drawn by these loads is rich in harmonics of the fundamental line frequency. For example, the lower-order harmonics, such as the third and fifth harmonic current components, can have very large magnitudes. Conventionally, the input section of rectifier/charger is a diode-bridge rectifier. The electrical characteristics at the input of this diode-bridge rectifier are identical to the input characteristics of the critical load. Therefore, the current drawn from the utility by the conventional UPS system comprises large amplitudes of harmonic currents. These harmonic currents can significantly interfere with other loads on the electric power line and with nearby telephone equipment. Additionally, these harmonics cause unnecessary heating of these other loads, and often contribute to a malfunction of ripple control systems within these other loads.
A variation of the above UPS system arrangement is known as a standby power supply (SPS) system. Under normal conditions, the SPS system delivers AC power directly from the utility to the critical load. During an emergency power outage, the SPS system supplies battery power through an inverter to the critical load. As mentioned above, the highly nonlinear input characteristic of the critical load draws a load current that is rich in harmonics of the fundamental line frequency. Since the critical load is supplied directly from the utility via the SPS system under normal conditions, the current drawn from the utility by the SPS system has large amounts of harmonic components. Therefore, both the conventional UPS and SPS systems disadvantageously draw undesirable harmonic current components from the utility system.
Thus a need exists for an improved UPS system, and more particularly for an improved SPS system, which is directed toward overcoming, and which is not susceptible to, the above limitations and disadvantages.